The present invention relates to a brushless electric machine comprising a stator with polyphase windings, a rotor having at least one permanent magnet magnetised radially relative to the axis of the rotor and a non-magnetic structure at least partially surrounding the magnet, and means for detecting the angular position of the rotor.
The invention is especially applicable to detection of the angular position of the rotor for automatic control of the variable frequency and amplitude polyphase supply of synchronous, permanent magnet electric motors which are provided with structural parts indicating the position of the rotor, where this supply can be sinusoidal or by way of pulses of continuous current. Motors of this kind are preferred in certain fields of application, such as medical or dental instruments because, in view of the reduced dimensions of the instruments, it is necessary to reduce as much as possible the size of the motor and the number of connections between the motor and its control device.
The object of the automatic control is in particular to obtain maximum torque and to avoid the synchronous motor pulling out of sync. Especially at low speeds it should allow good control over the torque and the speed and allow execution of special cycles of work, operating over a small number of revolutions or over fractions of a revolution of the rotor.
The conventional automatic control devices which use magnetic Hall sensors or an angular encoder for example to indicate the position of the rotor suffer from the problems of some bulk in the motor as well as supplementary electric connections between the motor and its control device. Other known devices uses voltage transformers to detect variations in the impedance of the stator windings, but these heavy and bulky components are also a problem.
It is known to use high frequency voltages applied to the stator windings to detect variations in the impedance of these windings as a function of the position of the magnetic field of the rotor. For example the document JP 9-163788 A provides an estimate of the position of the rotor when stopped on the basis of three high frequency voltages measured on the three phases of the stator, in order to be able to control starting of the motor with maximum torque and efficiency.
However, the inventor has found that, when a stator winding is connected in series in an oscillatory circuit and the frequency of resonance of the this circuit is observed, the frequency varies because of variations in the impedance of the circuit, which are related in particular to variations in the magnetic flux of the rotor in the stator windings. The angular position of the rotor can be calculated on the basis of the variations in this frequency. In French patent application No. 98 09153, published after the date of the present application, the use of two oscillatory circuits having different frequencies is described, to calculate the angular position of the moving rotor and in consequence to control the supply pulses of the motor.
The inventor has found that a rotor fitted with a sintered permanent magnet by itself causes a variation of frequency of the order of 10% as a function of its angular position relative to the stator windings passing this frequency. This variation is attributed to the preferentially aligned molecular structure, i.e. the anisotropy, resulting from the manufacture of the magnet. Unfortunately the variation in frequency thus obtained is relatively weak and partially random. Although it can be used, this variation does not allow of great precision.
One object of the present invention is to improve an electric machine of the kind specified above, in such as way as to amplify the variation in the frequency of resonance of the oscillatory circuit and thus to allow easier and more precise detection of the angular position of the rotor, whether it is moving or stationary.
To this end the electric machine according to the invention is characterized in that the said non-magnetic structure has zones of different thicknesses or of different permeabilities around the circumference of the magnet.
In other words, the radial gap between the peripheral surface of the rotor magnet and the internal surface of the stator pole pieces, which comprises the air-gap and the said non-magnetic structure, provides conditions for the passage of the magnetic flux which are variable around the circumference of the rotor. These conditions depend on both the nature and the thickness of the layers present in this gap, including the air-gap.
The result is that the high frequency magnetic flux produced through the rotor by the stator windings by virtue of the oscillatory circuit varies with the angular position of the rotor, not only because of the nature of the magnet, but in addition because of the form of the non-magnetic structure surrounding it. Appropriate choice of the geometry of this structure relative to the poles of the magnet makes it possible to augment the desired effect. The said zones of different thicknesses or permeabilities are preferably disposed symmetrically relative to a plane perpendicular to the direction of magnetisation of the magnet and passing through the axis of the rotor.
In small motors of this kind it is well known to fit the rotor magnet within a sleeve of non-magnetic material, the purpose of which is above all to prevent the magnet blowing apart under the influence of centrifugal force. In medical applications, which in particular require sterilisation procedures, the metal which is used is generally brass, because of its ease in machining and its good resistance to corrosion. In one advantageous embodiment of the invention, the non-magnetic structure of the rotor comprises such a sleeve, having recesses which form openings or thinned parts of the sleeve. These recesses can advantageously be filled with a dielectric material, an epoxy resin for example.
It should be noted that an electric machine according to the invention may be either a motor, or a generator, or a machine able to operate in both modes.
In one advantageous embodiment the means for detecting the angular position of the rotor comprise measuring means arranged to be connected selectively to windings of the stator in such a manner as to form an oscillatory circuit and to detect variations in the frequency of resonance of the said oscillatory circuit. However, it is important to note that the principles of the present invention can also be used with other types of detecting means. In this respect the eddy currents induced in the metal sleeve of the rotor play a major role in the methods of detection to be employed.